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SLA_PRECES - Precession   

ACTION:
Precession - either the old ``FK4'' (Bessel-Newcomb, pre IAU 1976) or new ``FK5'' (Fricke, post IAU 1976) as required.

CALL:
CALL sla_PRECES (SYSTEM, EP0, EP1, RA, DC)

GIVEN:

SYSTEM C precession to be applied: `FK4' or `FK5'
EP0,EP1 D starting and ending epoch
RA,DC D $[\,\alpha,\delta\,]$, mean equator & equinox of epoch EP0

RETURNED:

RA,DC D $[\,\alpha,\delta\,]$, mean equator & equinox of epoch EP1

NOTES:
1.
Lowercase characters in SYSTEM are acceptable.
2.
The epochs are Besselian if SYSTEM=`FK4' and Julian if `FK5'. For example, to precess coordinates in the old system from equinox 1900.0 to 1950.0 the call would be:
CALL sla_PRECES ('FK4', 1900D0, 1950D0, RA, DC)
3.
This routine will NOT correctly convert between the old and the new systems - for example conversion from B1950 to J2000. For these purposes see sla_FK425, sla_FK524, sla_FK45Z and sla_FK54Z.
4.
If an invalid SYSTEM is supplied, values of -99D0,-99D0 are returned for both RA and DC.


next up previous
Next: SLA_PRECL - Precession Matrix (latest)
Up: SUBPROGRAM SPECIFICATIONS
Previous: SLA_PREC - Precession Matrix (FK5)

SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk